GEOCHEMISTRY OF GARNET PYROXENITE LAYERS FROM THE EXTERNAL LIGURIDE MANTLE PERIDOTITES (NORTHERN APENNINE, ITALY): EVIDENCE FOR RECYCLING OF CRUSTAL MATERIAL IN A OPHIOLITIC MANTLE SEQUENCE

Abstract

The mantle peridotite bodies from the External Liguride ophiolites (Northern Apennine) preserve a subcontinental lithospheric origin and are locally characterised by the occurrence of garnet-bearing clinopyroxenite to websterite layers. Some of these layers preserve relics of a pristine garnet- bearing mineral assemblage, which yielded a thermobarometric equilibration at 1150-1200 °C and 2.6-3.0 GPa (Montanini et al., 2005). The garnet-bearing mantle section subsequently underwent a subsolidus decompression evolution to spinel- and plagioclase-facies conditions (see also Beccaluva et al., 1984). The garnet clinopyroxenite layers reach up to a couple of metres in thickness and may be subdivided into two groups. Group 1 clinopyroxenites have lower mg#, Cr and Ni contents than group 2 clinopyroxenites, and locally preserve accessory amounts of graphite. Group 1 clinopyroxenites display low SiO2/Al2O3 values and a REE pattern characterised by marked LREE depletion and weak positive Eu anomaly. Relict clinopyroxenes from these clinopyroxenites show a bell-shaped REE pattern, with a slight positive Eu anomaly. A weak positive Eu anomaly is also observed for associated garnets, which are characterised by a marked enrichment of MREE and HREE over LREE. Group 2 clinopyroxenites, as well as included clinopyroxenes, show variable LREE fractionation and positive Eu anomaly. The websterite layers are up to 8 cm in thickness. This rock type has been also found along the contacts between Group 2 clinopyroxenites and peridotites. Relative to the clinopyroxenites, websterites differ in the higher mg# (similar to those of the peridotites), Cr and Ni, and in the lower Al2O3, CaO and Na2O. The REE pattern of websterites is characterised by LREE depletion and lack of significant Eu anomaly. The peridotites enclosing the pyroxenite layers are plagioclase- bearing mylonites with relics of a spinel-facies, Tirich amphibole-bearing tectonitic assemblage. These mantle rocks have a fertile geochemical signature, as shown by the analyses of relict spinel-facies minerals. For instance, spinel cores and porphyroclastic clinopyroxenes have low cr# values and rather high Al2O3, Na2O and LREE contents, respectively. Other peridotite bodies from the External Liguride ophiolites locally retain similar compositional features (Rampone et al., 1995; Piccardo et al., 2004). Major and trace element compositions indicate that the garnet clinopyroxenites were derived from plagioclase-rich mafic cumulates. We also propose that at least Group 1 clinopyroxenites underwent an event of partial melting, in the garnet stability field. The websterites likely record interaction processes with the enclosing peridotites which partially obscures their origin. Two major hypotheses may be formulated for their petrogenesis, i.e. recycling of crustal protoliths variably modified by partial melting and/or diffusive mixing with the peridotite, or infiltration of partial melts derived from the nearby clinopyroxenites. Garnet and clinopyroxene separates from the pyroxenite layers have provided oxygen isotope values close to typical mantle. On the other hand, Sm-Nd isotope data argue against an origin from typical MORB-type cumulates, thus implying that the garnet pyroxenite layers from the External Liguride ophiolites do not simply represent oceanic crust recycled through an ancient subduction.